US20090213718A1 - Optical recording device and optical recording and reproduction device - Google Patents
Optical recording device and optical recording and reproduction device Download PDFInfo
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- US20090213718A1 US20090213718A1 US12/266,285 US26628508A US2009213718A1 US 20090213718 A1 US20090213718 A1 US 20090213718A1 US 26628508 A US26628508 A US 26628508A US 2009213718 A1 US2009213718 A1 US 2009213718A1
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1381—Non-lens elements for altering the properties of the beam, e.g. knife edges, slits, filters or stops
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/0465—Particular recording light; Beam shape or geometry
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/004—Recording, reproducing or erasing methods; Read, write or erase circuits therefor
- G11B7/0065—Recording, reproducing or erasing by using optical interference patterns, e.g. holograms
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1372—Lenses
- G11B7/1378—Separate aberration correction lenses; Cylindrical lenses to generate astigmatism; Beam expanders
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H2222/00—Light sources or light beam properties
- G03H2222/35—Transverse intensity distribution of the light beam
Definitions
- the present invention relates to an optical recording device and an optical recording and reproduction device.
- the coaxial recording method (collinear method) that has advantages in that its optical system can be significantly simplified in comparison to the conventional two-beam interference method, it is resistant to external disturbance such as vibration, and the introduction of a servo mechanism is easy.
- this collinear method signal light and reference light that have been modulated and generated by a spatial light modulator are collected with the same optical axis by the same lens, and an interference fringe (diffraction grating) that is formed by the interference between the signal light and the reference light is recorded as a hologram on an optical recording medium.
- a signal light pattern in which digital data have been two-dimensionally encoded is displayed on the spatial light modulator, whereby the digital data are superposed on the signal light.
- the optical recording medium on which the hologram has been recorded is irradiated with the reference light as reading light, whereby the signal light is reproduced from the recorded hologram. From this reproduced signal light, the superposed digital data can be decoded.
- the signal light and the reference light are on the same optical axis, when the optical recording medium is irradiated with the reference light as reading light during reproduction, light leaking to the region of the spatial light modulator that corresponds to the signal light.
- the precision of liquid crystal elements that have been disposed as a transmissive spatial light modulator is low, the light that has been transmitted through the OFF pixels that are in display positions of the signal light pattern becomes leak light. Consequently, when all of the pixels of the spatial light modulator are irradiated with light in this manner, this leak light is detected by a photodetector, noise with respect to the reproduced signal light is generated, and reproduction characteristics deteriorate.
- an optical recording device comprises a light source, a spatial light modulator, a correcting optical system, and a light guiding section.
- the light source emits coherent light.
- the spatial light modulator is formed from a plurality of pixels that are two-dimensionally arrayed, comprises a signal light region that displays a signal light pattern when generating signal light, and a reference light region that is disposed so as to surround the signal light region and displays a reference light pattern when generating reference light that is coaxial with the signal light, and modulates and outputs incident light for each pixel in accordance with a display pattern.
- the correcting optical system is disposed between the light source and the spatial light modulator, and comprises a pair of axicon lenses that correct the light that has been emitted from the light source so as to flatten the light intensity distribution thereof on an irradiated surface of the spatial light modulator.
- the light guiding section guides the light that has been corrected by the correcting optical system to the spatial light modulator.
- the signal light and the reference light generated by the spatial light modulator are irradiated onto an optical recording medium simultaneously and a hologram is recorded into the optical recording medium.
- FIG. 1 is a schematic diagram showing the configuration of an optical recording and reproduction device pertaining to the exemplary embodiment of the present invention
- FIG. 2A is a diagram showing an example of a recording pattern that is displayed on a spatial light modulator during recording
- FIG. 2B is a plane view showing an example of a display region that is set on a display screen of the spatial light modulator
- FIG. 3A is a schematic diagram showing an example of the configuration of an axicon optical system
- FIG. 3B and FIG. 3C are schematic diagrams showing an example of a diaphragm mechanism
- FIG. 4A is a diagram showing beam correction operation of the axicon optical system during recording
- FIG. 4B is a diagram showing beam correction operation of the axicon optical system during reproduction
- FIG. 5A is a diagram for describing an optical condition where circular incident parallel light with a diameter D is emitted as circular parallel light with the diameter D;
- FIG. 5B is a diagram showing a result of simulating a light intensity distribution being flattened by a pair of axicon lenses
- FIG. 6A to FIG. 6D are diagrams showing results when the effect of flattening the light intensity distribution by the pair of axicon lenses has been simulated
- FIG. 7 is a diagram showing the arrangement of the respective lenses that configure the pair of axicon lenses
- FIG. 8A is a perspective diagram showing the display surface of the spatial light modulator being irradiated with laser light during recording
- FIG. 8B is a perspective diagram showing the display surface of the spatial light modulator being irradiated with laser light during reproduction;
- FIG. 9A and FIG. 9B are diagrams showing the effect of introducing the axicon optical system
- FIG. 10A is a histogram of ON pixels (white pixels) of a data page shown in FIG. 9A ;
- FIG. 10B is a histogram of ON pixels of a data page shown in FIG. 9B ;
- FIG. 11 is a schematic diagram showing the configuration of a coaxial reflective optical recording and reproduction device to which the axicon optical system has been introduced.
- FIG. 1 is a schematic diagram showing the configuration of an optical recording and reproduction device pertaining to the exemplary embodiment of the present invention.
- This optical recording and reproduction device is a “coaxial recording method (collinear method)” optical recording and reproduction device that irradiates an optical recording medium with signal light and reference light that share an optical axis as recording light of one light beam from the same direction.
- a “coaxial transmissive” optical recording and reproduction device that uses a reflective spatial light modulator (SLM) and a transmissive optical recording medium will be described.
- SLM spatial light modulator
- a light source 10 that emits laser light that is coherent light.
- the light source 10 for example, a laser light source that emits green laser light whose emission wavelength is 532 nm is used.
- a shutter 12 that is capable of inserting into and withdrawing from (opening and closing) the optical path, a 1 ⁇ 2 wavelength plate 14 that applies a phase difference of 1 ⁇ 2 wavelength between linearly polarized components that are orthogonal, a polarizing plate 16 that allows light of a predetermined polarization direction to pass therethrough, a beam expander 18 that is an expanding/collimating optical system, an axicon optical system 20 that is used as a correcting optical system and a reflecting mirror 22 are disposed in this order along the optical path from the side of the light source 10 .
- the shutter 12 is driven to open and close by a drive device (not shown) that is connected to a control device (not shown) such as a computer.
- a polarizing beam splitter 24 that reflects light of a predetermination polarization direction and transmits light of a polarization direction that is orthogonal thereto.
- a reflective spatial light modulator 26 that polarizes and modulates incident light per pixel.
- the reflective spatial light modulator 26 there can be used liquid crystal on silicon (LCOS) or the like.
- the spatial light modulator 26 is connected to a control device (not shown) via a pattern generator (not shown). Each of the pixel portions of the spatial light modulator 26 is driven and controlled by this control device.
- the pattern generator applies two-dimensional encoding to digital data that have been supplied from the control device to generate a signal light pattern that is displayed on the spatial light modulator 26 .
- the signal light pattern is, for example, a digital pattern where binary digital data “0, 1” are expressed as “dark (black pixels), bright (white pixels)”.
- a reference light pattern is also displayed on the spatial light modulator 26 .
- the reference light pattern is, for example, a random pattern.
- the spatial light modulator 26 modulates incident laser light in accordance with the displayed signal light pattern and reference light pattern to generate signal light and reference light.
- the spatial light modulator 26 reflects the generated signal light and reference light toward the polarizing beam splitter 24 .
- FIG. 2A is a diagram showing an example of a recording pattern that is displayed on the spatial light modulator 26 during recording.
- a recording pattern 64 includes a signal light pattern 66 that generates signal light and a ring-shaped reference light pattern 68 that generates reference light.
- the signal light pattern 66 is displayed in the central portion of the spatial light modulator 26 .
- the reference light pattern 68 is displayed on the peripheral portion of the spatial light modulator 26 so as to surround this signal light pattern 66 .
- the region that displays the signal light pattern is a signal light region, and the region that displays the reference light pattern is a reference light region.
- the shape of the signal light region and the shape of the reference light region can be appropriately altered in accordance with the recording pattern.
- FIG. 2B is a plane view showing an example of a display region that is set on a display screen 26 A of the spatial light modulator 26 .
- the size of the recording pattern and the pattern disposition are set beforehand in accordance with the size and the like of the display screen 26 A.
- on the display screen 26 A of the spatial light modulator 26 there are respectively disposed a circular signal light region 26 S and a ring-shaped reference light region 26 R that surrounds the signal light region 26 S.
- the axicon optical system 20 is a correcting optical system at least which, during recording, corrects incident parallel light from the beam expander 18 so as to flatten its light intensity distribution on the display screen 26 A of the spatial light modulator 26 and generates parallel light with which the spatial light modulator 26 is irradiated. Further, during reproduction, the axicon optical system 20 corrects incident parallel light from the beam expander 18 such that the signal light region 26 S of the spatial light modulator 26 is not irradiated with laser light and generates parallel light with which the spatial light modulator 26 is irradiated.
- the axicon optical system 20 during recording, the axicon optical system 20 generates parallel light whose cross-sectional shape that is orthogonal to the optical path is circular (below, called “circular parallel light”) in order to irradiate the circular recording region that includes the signal light region 26 S and the reference light region 26 R with laser light. Further, during reproduction, the axicon optical system 20 generates parallel light whose cross-sectional shape that is orthogonal to the optical path is ring-shaped (below, called “ring-shaped parallel light”) in order to irradiate the ring-shaped reference light region 26 R with laser light. It will be noted that the axicon optical system 20 will be described in detail later.
- the signal light and the reference light that have been generated by the spatial light modulator 26 are made incident.
- a pair of lenses 28 and 32 and a Fourier transform lens 34 are disposed in this order along the optical path.
- the Fourier transform lens 34 Fourier-transforms recording light and irradiates an optical recording medium 36 with the Fourier-transformed recording light.
- the focal position of the Fourier transform lens 34 becomes a light collecting position where the recording light is collected.
- a light blocking plate 30 that includes an opening portion (an aperture) 30 A. It will be noted that the light blocking plate 30 is not essential and can be appropriately omitted.
- a holding stage (not shown) that holds the optical recording medium 36 .
- the holding stage is driven by a drive device (not shown) that is connected to a control device (not shown) and moves in the optical axis direction or a plane direction that is perpendicular to the optical axis.
- the holding stage for example, holds the optical recording medium 36 in a standard position where the film-thickness direction center position of the optical recording medium 36 becomes the focal position of the Fourier transform lens 34 .
- the optical recording medium 36 is an optical recording medium on which a hologram is capable of being recorded by a change in the index of refraction resulting from being irradiated with light.
- Examples of the optical recording medium 36 may include an optical recording medium that uses a recording material such as a photopolymer material, a photorefractive material or a silver halide photosensitive material.
- the sensor array 46 is configured by imaging elements such as CCDs or a CMOS array, converts reproduced light (diffracted light) that has been received into electrical signals and outputs the electrical signals. Between the lens 38 and the lens 42 , there is disposed a light blocking plate 40 that includes an opening portion (an aperture) 40 A with a large opening diameter. It will be noted that the light blocking plate 40 is not essential and can be appropriately omitted. Further, the sensor array 46 is connected to a control device (not shown). During reproduction, the sensor array 46 reads data that have been superposed on signal light that has been reproduced and outputs the data to a control device (not shown).
- FIG. 3A is a schematic diagram showing an example of the configuration of the axicon optical system 20 .
- the axicon optical system 20 is configured to include a mask 50 that comprises a light transmitting plate and includes a blocking portion 50 A that blocks light in the vicinity of an optical axis L, a diaphragm mechanism (an iris) 56 that includes an opening portion 56 A whose diameter changes to adjust the beam diameter of the light that passes therethrough, and a pair of axicon lenses 58 .
- Laser light is made incident on the axicon optical system 20 from the beam expander 18 .
- the mask 50 , the diaphragm mechanism 56 and the pair of axicon lenses 58 are disposed in this order along the optical path from the laser light incident side.
- the diaphragm mechanism 56 can, as shown in FIG. 3B and FIG. 3C , for example, be configured by superimposing plural plates (aperture blades) 56 B. Each of the plural aperture blades 56 B changes the diameter of the opening portion 56 A by fixing and simultaneously rotating one end. As shown in FIG. 3B , during recording, in a state where the diameter of the opening portion 56 A has expanded, light with a large opening diameter is allowed to pass therethrough. On the other hand, as shown in FIG. 3C , during reproduction, the diameter of the opening portion 56 A contracts toward the optical axis from the outside along the direction of arrow A such that the beam diameter of the light that passes through the opening portion 56 A becomes smaller. It will be noted that, when an automatic diaphragm mechanism or the like that uses an ultrasonic motor is used as the diaphragm mechanism 56 , high-speed driving becomes possible.
- FIG. 7 is a diagram showing the arrangement of the respective lenses that configure the pair of axicon lenses 58 .
- the pair of axicon lenses 58 are configured by two lenses that have the same shape: an axicon lens 58 A and an axicon lens 58 B.
- An axicon is an optical device that is disposed with an axisymmetric surface that converts light from a point light source into a linear image on an optical axis.
- an axicon is used in the configuration of a conical lens that is disposed with a conical surface, and a truncated cone-shaped lens where one surface is a flat surface and the other surface is a conical surface is usually called an axicon lens.
- the truncated cone-shaped lenses shown in FIG. 7 are called axicon lenses.
- the axicon lens 58 A and the axicon lens 58 B are disposed apart from each other by a predetermined distance such that their conical axes coincide with the optical axis L (i.e., such that each of the apexes of the cones is positioned on the same optical axis) and such that their conical surfaces face each other.
- the distance by which the axicon lens 58 A and the axicon lens 58 B are disposed apart from each other will be described in detail later.
- FIG. 4A is a diagram showing beam correction operation of the axicon optical system 20 during recording
- FIG. 4B is a diagram showing beam correction operation of the axicon optical system 20 during reproduction.
- the light intensity distribution of the circular parallel light with the diameter D that has passed through the opening portion 56 A in the diaphragm mechanism 56 is flattened by the axicon lenses 58 , and the circular parallel light with the diameter D is emitted as circular parallel light 100 with the same diameter D. That is, when the circular parallel light with the diameter D is made incident from the flat surface side of the axicon lens 58 A, when the incident light is emitted from the slanted surface and the apex of the conical surface, it is refracted as in FIG. 4A and a circular beam is formed.
- the circular beam is made incident from the conical surface of the axicon lens 58 B, is formed into the circular parallel light 100 with the diameter D, and is emitted from the flat surface side of the axicon lens 58 B. That is, it fulfills the role of folding back the light in the vicinity of the optical axis of the incident light to the outside and of folding back the light in the vicinity of the circumference to the vicinity of the optical axis.
- Laser light has a Gaussian intensity distribution that has a peak on its optical axis, so by spreading the light in the vicinity of the optical axis where its intensity is high to the outside and causing the light in the vicinity of the circumference where its intensity is low to be concentrated in the vicinity of the optical axis, it becomes possible to make the light intensity distribution uniform.
- the diameter D of the circular parallel light 100 is designed so as to become equal to the outer diameter of the reference light region 26 R.
- the signal light region 26 S and the reference light region 26 R shown in FIG. 2B are exactly irradiated with the circular parallel light 100 .
- the mask 50 that includes the blocking portion 50 A is irradiated with the circular parallel light with the diameter D that has been made incident on the axicon optical system 20 .
- the parallel light with which the mask 50 has been irradiated the light intensity component that would be transmitted through the apexes of the axicon lenses 58 A and 58 B and travel straightly is removed by the blocking portion 50 A, and the remaining portion passes through the mask 50 .
- the diaphragm mechanism 56 is irradiated with the parallel light that has passed through the mask 50 .
- the diameter of the opening portion 56 A in the diaphragm mechanism 56 contracts.
- the diameter of the opening portion 56 A at this time is d ( ⁇ D).
- the outside of the circular parallel light with the diameter D with which the diaphragm mechanism 56 has been irradiated is cut by the diaphragm mechanism 56 such that just circular parallel light with the diameter d passes through the opening portion 56 A.
- the circular parallel light with the diameter d that has passed through the opening portion 56 A in the diaphragm mechanism 56 is converted from a circular beam into a ring-shaped beam by the pair of axicon lenses 58 such that ring-shaped parallel light 102 is emitted.
- the circular parallel light with the diameter d is made incident from the flat surface side of the axicon lens 58 A
- the incident light is emitted from the slanted surface and the apex of the conical surface
- it is refracted as in FIG. 4B and a ring-shaped beam is formed.
- the ring-shaped beam is made incident from the conical surface of the axicon lens 58 B, is formed into the ring-shaped parallel light 102 , and is emitted from the flat surface side of the axicon lens 58 B.
- the outer diameter (diameter of the outer circumference) of the ring-shaped parallel light 102 that is emitted is D
- the inner diameter (diameter of the inner circumference) of the ring-shaped parallel light 102 that is emitted is D in (see FIG. 7 ).
- the outer diameter D and the inner diameter D in are determined in accordance with the diameter d of the incident parallel light, a diameter A of the axicon lenses, an angle ⁇ of the apex angle of the conical portions and a distance L by which the pair of axicon lenses are disposed apart from each other.
- the distance L by which the pair of axicon lenses are disposed apart from each other will be described later.
- the outer diameter D and the inner diameter D in of the ring-shaped parallel light 102 are designed such that the outer diameter D becomes equal to the outer diameter of the reference light region 26 R and such that the inner diameter D in becomes equal to the inner diameter of the reference light region 26 R.
- the reference light region 26 R shown in FIG. 2B is exactly irradiated with the ring-shaped parallel light 102 .
- the inner diameter of the reference light region 26 R and the diameter of the signal light region 26 S are described as being equal, but from the standpoint of reducing leak light that leaks to the region that corresponds to the signal light during reproduction, it is preferable to make the inner diameter of the reference light region 26 R larger than the diameter of the signal light region 26 S. That is, it is preferable to dispose a gap between the reference light region 26 R and the signal light region 26 S. Further, from the same standpoint, it is more preferable to make the inner diameter D in of the ring-shaped parallel light 102 slightly larger than the inner diameter of the reference light region 26 R.
- FIG. 5A is a diagram for describing an optical condition for the circular incident parallel light with the diameter D to be emitted as circular parallel light with the same diameter D as shown in FIG. 4A .
- the distance L by which the axicon lens 58 A and the axicon lens 58 B are disposed apart from each other is defined as an optical path component distance of the optical path length between the axicon lens 58 A and the axicon lens 58 B.
- FIG. 5B is a diagram showing a result of simulating incident light being refracted by the pair of axicon lenses 58 with a wave-optics technique.
- Arrow B represents the waveguide direction of the light.
- the light intensity distribution is converted while the parallel light passes through the pair of axicon lenses 58 , and parallel light is generated whose light intensity distribution in its cross section that is orthogonal to the optical axis is uniform.
- FIG. 6A to FIG. 6D are diagrams showing results when the effect of flattening the light intensity distribution by the pair of axicon lenses has been simulated with a wave-optics technique.
- the axicon lens 58 A and the axicon lens 58 B are disposed apart from each other by the distance L so as to satisfy the above expression. Further, the above-described mask 50 is not used.
- FIG. 6A and FIG. 6C are diagrams showing the beam profile of the parallel light before being made incident on the pair of axicon lenses 58 .
- FIG. 6B and FIG. 6D are diagrams showing the beam profile of the parallel light that has been emitted from the pair of axicon lenses 58 .
- the horizontal axis represents distance (unit: mm) from a standard position, and the optical axis is in a position of 7 mm from the standard position.
- the vertical axis represents a relative light intensity A when a maximum light intensity is 1.
- the beam that has a Gaussian intensity distribution in the beam that has a Gaussian intensity distribution, the light intensity in the central portion (in the vicinity of the optical axis) is large, and the light intensity decreases toward the periphery.
- the beam that has passed through the pair of axicon lenses 58 as shown in FIG. 6B and FIG. 6D , the light intensity in the central portion is distributed to the peripheral portion, the light intensity at the peripheral portion increases, and the light intensities are substantially the same between the central portion and the peripheral portion.
- the beam profile shown in FIG. 6D has a substantially trapezoidal shape.
- the beam that has a Gaussian intensity distribution is converted into a beam whose light intensity is flattened and whose light intensity distribution in its cross section that is orthogonal to the optical axis is uniform.
- the mask 50 that includes the blocking portion 50 A is disposed on the light incident side of the pair of axicon lenses 58 to thereby block the light intensity component that would be transmitted through the apexes of the axicon lenses 58 and travel straightly.
- the turbulence in the light intensity distribution is removed at the central portion, and the light intensity distribution is made even more uniform.
- the mask 50 can also be appropriately omitted by a contrivance such as not using the signal light region in the vicinity of the center where the light intensity distribution becomes turbulent. It will be noted that the conditional expression in the present invention has been described by a case where the mask 50 is omitted. When the mask 50 is introduced, the conditional expression may be appropriately corrected in accordance with the size of the blocking portion 50 A.
- the axicon optical system 20 generates the circular parallel light 100 with the diameter D whose light intensity distribution has been flattened in order to irradiate, with laser light, the circular recording region that includes the signal light region 26 S and the reference light region 26 R shown in FIG. 2B . Further, during reproduction, the axicon optical system 20 generates ring-shaped parallel light in order to irradiate, with laser light, just the ring-shaped reference light region 26 R of the spatial light modulator 26 . That is, during reproduction, the axicon optical system 20 generates ring-shaped parallel light such that the signal light region 26 S of the spatial light modulator 26 is not irradiated with laser light.
- the parallel light generated by the axicon optical system 20 is, as mentioned later, reflected by the reflecting mirror 22 and reflected by the polarizing beam splitter 24 such that the spatial light modulator 26 is irradiated therewith.
- the signal light region 26 S and the reference light region 26 R are irradiated with the circular parallel light 100 and, during reproduction, the reference light region 26 R is irradiated with the ring-shaped parallel light 102 .
- the shutter 12 When a hologram is to be recorded, the shutter 12 is opened and laser light is emitted from the light source 10 . At the same time, a recording pattern is displayed on the spatial light modulator 26 .
- the laser light that has been emitted from the light source 10 passes through the shutter 12 , and its light intensity and polarization direction are adjusted by the 1 ⁇ 2 wavelength plate 14 and the polarizing plate 16 .
- the polarizing plate 16 has an arrangement that transmits only S-polarized light, and the polarization direction of the laser light is controlled by the 1 ⁇ 2 wavelength plate 14 , whereby the light intensity of the S-polarized light is adjusted.
- the light that has passed through the polarizing plate 16 is converted into parallel light with a large diameter by the beam expander 18 , and circular parallel light with the diameter D is made incident on the axicon optical system 20 .
- the circular incident parallel light with the diameter D is corrected, and the circular parallel light 100 with the diameter D whose light intensity distribution has been flattened is generated.
- the reflecting mirror 22 is irradiated with the parallel light that has been emitted from the axicon optical system 20 .
- the parallel light that has been reflected by the reflecting mirror 22 is made incident on the polarizing beam splitter 24 .
- the polarizing beam splitter 24 reflects S-polarized light and transmits P-polarized light.
- the circular parallel light 100 with the diameter D (S-polarized light) is reflected by the polarizing beam splitter 24 in the direction of the spatial light modulator 26 .
- the recording pattern is displayed.
- the laser light is polarized and modulated (from S-polarized light to P-polarized light) in accordance with the displayed pattern, and signal light and reference light are generated.
- the signal light region 26 S and the reference light region 26 R of the spatial light modulator 26 are irradiated with the circular parallel light 100 generated by the axicon optical system 20 .
- the laser light that has been made incident on the signal light region 26 S is polarized and modulated in accordance with the displayed signal light pattern, and signal light is generated.
- the laser light that has been made incident on the reference light region 26 R is polarized and modulated in accordance with the displayed reference light pattern, and reference light is generated. Because the light intensity distribution of the circular parallel light 100 is flattened, the recording pattern that is displayed on the spatial light modulator 26 can be irradiated uniformly.
- the SNR of the signal light that is generated improves, and a hologram is recorded in a high SNR.
- the recording light that has been polarized and modulated by the spatial light modulator 26 is emitted to the polarizing beam splitter 24 , passes through the polarizing beam splitter 24 , and is converted into an amplitude distribution of linearly polarized light (P-polarized light). Thereafter, the recording light is collected by the lens 28 , and the light blocking plate 30 that includes the aperture 30 A is irradiated therewith. The unnecessary frequency component of the recording light that has been collected by the lens 28 is cut by the light blocking plate 30 , and the remaining portion passes through the aperture 30 A. The recording light that has passed through the aperture 30 A is converted into parallel light by the lens 32 .
- the recording light that has been converted into parallel light by the lens 32 that is, the signal light and the reference light are Fourier-transformed and collected by the Fourier transform lens 34 , and the optical recording medium 36 is simultaneously and coaxially irradiated therewith.
- an interference fringe that is formed as a result of the signal light and the reference light interfering with each other is recorded as a hologram on the optical recording medium 36 .
- the shutter 12 When data that have been recorded on the optical recording medium 36 are to be read (during reproduction), the shutter 12 is opened and laser light is emitted from the light source. At the same time, a reproduction pattern is displayed on the spatial light modulator 26 .
- the laser light that has been emitted from the light source 10 in the same manner as in the case of recording, passes through the shutter 12 , its light intensity and polarization direction are adjusted by the 1 ⁇ 2 wavelength plate 14 and the polarizing plate 16 , it is converted into parallel light with a large diameter by the beam expander 18 , and it is made incident on the axicon optical system 20 .
- the circular incident parallel light with the diameter D is corrected, and the ring-shaped parallel light 102 with the outer diameter D and the inner diameter d is generated.
- the light intensity distribution of the ring-shaped parallel light 102 is also flattened.
- the reflecting mirror 22 is irradiated with the ring-shaped parallel light 102 that has been emitted from the axicon optical system 20 .
- the ring-shaped parallel light 102 that has been reflected by the reflecting mirror 22 is made incident on the polarizing beam splitter 24 .
- the ring-shaped incident parallel light 102 (S-polarized light) is reflected by the polarizing beam splitter 24 in the direction of the spatial light modulator 26 .
- the reproduction pattern is displayed.
- the laser light is polarized and modulated (from S-polarized light to P-polarized light) in accordance with the displayed pattern, and reference light is generated.
- the reference light region 26 R of the spatial light modulator 26 is irradiated with the ring-shaped parallel light 102 that has been generated by the axicon optical system 20 .
- the laser light that has been made incident on the reference light region 26 R is polarized and modulated in accordance with the displayed reference light pattern, and reference light is generated.
- the region of the optical recording medium 36 on which the hologram has been recorded is, in the same manner as in the case of recording, irradiated with the generated reference light that. That is, the optical recording medium 36 is irradiated with just the reference light as reading light.
- the ring-shaped parallel light 102 is generated in accordance with the shape of the reference light region 26 R, just the reference light region 26 R is irradiated with the laser light, and the signal light region 26 S is not irradiated with the laser light. Consequently, the optical recording medium 36 is not irradiated with the light that has been reflected by the signal light region 26 S (leak light that is present in the region that corresponds to the signal light) together with the reference light for reading. Consequently, in the reading light with which the hologram is irradiated, leak light (unnecessary component) that leaks to the region that corresponds to the signal light is significantly reduced and reproduction characteristics remarkably improve.
- the reference light with which the optical recording medium 36 has been irradiated passes through the optical recording medium 36 , it is diffracted by the hologram, and the transmitted diffracted light (reproduced light) is emitted toward the Fourier transform lens 38 .
- Some of the reference light is transmitted through the optical recording medium 36 without being diffracted.
- the emitted reproduced light (including the transmitted diffracted light) is inverse-Fourier-transformed by the Fourier transform lens 38 , and the light blocking plate 40 that includes the aperture 40 A is irradiated therewith.
- the transmitted reference light is cut by the light blocking plate 40 and the remaining portion passes through the aperture 40 A.
- the light that has passed through the aperture 40 A is relayed by the pair of lenses 42 and 44 and is made incident on the sensor array 46 .
- the sensor array 46 converts the reproduced light that has been received into electrical signals and outputs the electrical signals. That is, the sensor array 46 reads the data that have been superposed on the reproduced signal light and outputs the data to a control device (not shown). It will be noted that, in the sensor array 46 , it is preferable to implement oversampling where one pixel of the signal light data is received by plural light receiving elements. For example, data of 1 bit are received by four (2 ⁇ 2) light receiving elements.
- FIG. 9A and FIG. 9B are diagrams showing the effect of introducing the axicon optical system.
- FIG. 9A is a diagram showing a result when the light intensity distribution of signal light (a data page) observed on the display surface 26 A of the spatial light modulator 26 has been simulated in a case where there is used an optical recording and reproduction device with the same configuration as has conventionally been the case excluding the axicon optical system from the configuration shown in FIG. 1 (below, called “the conventional device”).
- FIG. 9B is a diagram showing a result when the light intensity distribution of a data page observed on the display surface 26 A of the spatial light modulator 26 has been simulated in a case where there is used an optical recording and reproduction device with the same configuration as the configuration shown in FIG.
- the light intensity distributions of the data pages shown in FIG. 9A and FIG. 9B were obtained by multiplying the light intensity distributions shown in FIG. 6A and FIG. 6B that have been obtained by simulation and the intensity distribution of an ideal data pattern.
- the spatial light modulator is irradiated with the laser light with the Gaussian intensity distribution shown in FIG. 6A , so the central portion of the data page observed is bright but the peripheral portion is dark.
- the spatial light modulator is irradiated with the laser light with the flattened light intensity distribution shown in FIG. 6B , so a data page with uniform brightness is observed.
- the spatial light modulator is uniformly irradiated with the signal light pattern that is displayed on the spatial light modulator 26 , signal light (a data page) with a high SNR is generated.
- FIG. 10A is a histogram of ON pixels (white pixels) of the data page shown in FIG. 9A
- FIG. 10B is a histogram of ON pixels of the data page shown in FIG. 9B
- the horizontal axes represent brightness (in arbitrary units), and the vertical axes represent the number of occurrences (units: number of times) of ON pixels within the data pages.
- the brightness of the ON pixels distributes in the range of 8 to 40, and the SNR of the data page is 4.75.
- the SNR of the data page improves to 15.73, or greater than 3 times the SNR of the data page that was obtained in the conventional device.
- the fact that the range of the brightness distribution of the ON pixels is narrow means that the brightness of the ON pixels is substantially constant within the data page and that the light intensity distribution is flattened.
- SNR is defined as being equal to: (average value of brightness)/(standard deviation of brightness).
- the signal light region 26 S and the reference light region 26 R of the spatial light modulator 26 are irradiated with laser light.
- just the reference light region 26 R of the spatial light modulator 26 is irradiated with laser light.
- the minimum diffraction efficiency was evaluated using the diffraction efficiency when the bit error rate (BER) is equal to 5 ⁇ 10 ⁇ 3 as a minimum diffraction efficiency that is capable of being reproduced.
- BER bit error rate
- the minimum diffraction efficiency when reproduced by the device to which the axicon optical system has been introduced is 0.1%, and the minimum diffraction efficiency when reproduced by the conventional device is 1.0%. It will be understood that the minimum diffraction efficiency drops from 1.0% to 0.1% and that reproduction characteristics remarkably improve.
- the evaluations here were performed by simulation images that were obtained by imaging leak light of the signal light region when irradiated with the signal light and computationally adding the obtained images and the images that were obtained by simulation ( FIG. 9A and FIG. 9B ).
- FIG. 11 is a schematic diagram showing the configuration of a coaxial reflective optical recording and reproduction device to which the axicon optical system has been introduced. The same reference numerals will be given to configural portions that are the same as those of the optical recording and reproduction device pertaining to the preceding exemplary embodiment shown in FIG. 1 .
- the light source 10 that emits laser light.
- the shutter 12 On the light exiting side of the light source 10 , the shutter 12 , the 1 ⁇ 2 wavelength plate 14 , the polarizing plate 16 , the beam expander 18 , the axicon optical system 20 and the reflecting mirror 22 are disposed in this order along the optical path from the side of the light source 10 .
- the polarizing beam splitter 24 On the light reflecting side of the reflecting mirror 22 , there is disposed the polarizing beam splitter 24 .
- the reflective spatial light modulator 26 When seen from the reflecting mirror 22 side, on the light reflecting side of the polarizing beam splitter 24 , there is disposed the reflective spatial light modulator 26 .
- the spatial light modulator 26 modulates incident laser light in accordance with the displayed signal light pattern and reference light pattern to generate signal light and reference light.
- the spatial light modulator 26 reflects the generated signal light and reference light toward the polarizing beam splitter 24 .
- the axicon optical system 20 has the same configuration as that of the preceding exemplary embodiment.
- the axicon optical system 20 is, as shown in FIG. 3A , configured to include the mask 50 that includes the blocking portion 50 A, the diaphragm mechanism 56 and the pair of axicon lenses 58 .
- Laser light is made incident on the axicon optical system 20 from the beam expander 18 .
- the mask 50 , the diaphragm mechanism 56 and the pair of axicon lenses 58 are disposed in this order along the optical path from the laser light incident side.
- the signal light and the reference light that have been generated by the spatial light modulator 26 are made incident.
- a pair of lenses 84 and 88 a 1 ⁇ 4 wavelength plate 90 that converts linearly polarized light into circularly polarized light and which converts circularly polarized light into linearly polarized light and a Fourier transform lens 92 are disposed in this order along the optical path.
- the Fourier transform lens 92 irradiates a reflective optical recording medium 94 with recording light.
- the focal position of the Fourier transform lens 92 becomes a light collecting position where the recording light is collected.
- a light blocking plate 86 that includes an opening portion (an aperture) 86 A. It will be noted that the light blocking plate 86 is not essential and can be appropriately omitted.
- the optical recording medium 94 is an optical recording medium on which a hologram is capable of being recorded by a change in the index of refraction resulting from being irradiated with light, and the optical recording medium 94 is disposed with a recording layer 94 A that is configured by a recording material such as a photopolymer material on which a hologram is capable of being recorded and a reflective layer 94 B that is configured by a metal film or the like that reflects light that is transmitted through the recording layer 94 A.
- the sensor array 96 When seen from the lens 84 , on the light reflecting side of the polarizing beam splitter 24 , there is disposed a sensor array 96 .
- the sensor array 96 is configured by imaging elements such as CCDs or a CMOS array, converts reproduced light (diffracted light) that has been received into electrical signals and outputs the electrical signals.
- the shutter 12 When a hologram is to be recorded, the shutter 12 is opened and laser light is emitted from the light source 10 . At the same time, a recording pattern is displayed on the spatial light modulator 26 .
- the laser light that has been emitted from the light source 10 passes through the shutter 12 , and its light intensity and polarization direction are adjusted by the 1 ⁇ 2 wavelength plate 14 and the polarizing plate 16 .
- the light that has passed through the polarizing plate 16 is converted into parallel light with a large diameter by the beam expander 18 and is made incident on the axicon optical system 20 .
- the circular incident parallel light with the diameter D is corrected, and the circular parallel light 100 with the diameter D whose light intensity distribution has been flattened is generated.
- the reflecting mirror 22 is irradiated with the parallel light that has been emitted from the axicon optical system 20 .
- the parallel light that has been reflected by the reflecting mirror 22 is made incident on the polarizing beam splitter 24 .
- the circular parallel light 100 with the diameter D is reflected by the polarizing beam splitter 24 in the direction of the spatial light modulator 26 .
- the laser light is polarized and modulated in accordance with the displayed pattern, and signal light and reference light are generated.
- the recording light that has been polarized and modulated by the spatial light modulator 26 is emitted to the polarizing beam splitter 24 , passes through the polarizing beam splitter 24 , and is converted into an amplitude distribution of linearly polarized light. Thereafter, the recording light is collected by the lens 84 , and the light blocking plate 86 that includes the aperture 86 A is irradiated therewith. The unnecessary frequency component of the recording light that has been collected by the lens 84 is cut by the light blocking plate 86 , and the remaining portion passes through the aperture 86 A. The recording light that has passed through the aperture 86 A is converted into parallel light by the lens 88 .
- the recording light (the signal light and the reference light) that has been converted into parallel light by the lens 88 is converted into circularly polarized light by the 1 ⁇ 4 wavelength plate 90 and is Fourier-transformed and collected by the Fourier transform lens 92 , and the optical recording medium 94 is simultaneously and coaxially irradiated therewith.
- an interference fringe that is formed as a result of the signal light and the reference light interfering with each other is recorded as a hologram on the optical recording medium 94 .
- the shutter 12 When data that have been recorded on the optical recording medium 94 are to be read, the shutter 12 is opened and laser light is emitted from the light source. At the same time, a reproduction pattern is displayed on the spatial light modulator 26 .
- the laser light that has been emitted from the light source 10 in the same manner as in the case of recording, passes through the shutter 12 , its light intensity and polarization direction are adjusted by the 1 ⁇ 2 wavelength plate 14 and the polarizing plate 16 , it is converted into parallel light with a large diameter by the beam expander 18 , and it is made incident on the axicon optical system 20 .
- the circular incident parallel light with the diameter D is corrected, and the ring-shaped parallel light 102 with the outer diameter D and the inner diameter d is generated.
- the light intensity distribution of the ring-shaped parallel light 102 is also flattened.
- the reflecting mirror 22 is irradiated with the ring-shaped parallel light 102 that has been emitted from the axicon optical system 20 .
- the ring-shaped parallel light 102 that has been reflected by the reflecting mirror 22 is made incident on the polarizing beam splitter 24 .
- the ring-shaped incident parallel light 102 is reflected by the polarizing beam splitter 24 in the direction of the spatial light modulator 26 .
- the laser light is polarized and modulated in accordance with the displayed reproduction pattern, and reference light is generated.
- the region of the optical recording medium 94 on which the hologram has been recorded is, in the same manner as in the case of recording, irradiated with the generated reference light. That is, the optical recording medium 94 is irradiated with just the reference light as reading light.
- the ring-shaped parallel light 102 is generated in accordance with the shape of the reference light region 26 R, just the reference light region 26 R is irradiated with the laser light, and the signal light region 26 S is not irradiated with the laser light. Consequently, the optical recording medium 94 is not irradiated with the light that has been reflected by the signal light region 26 S (leak light that is present in the region that corresponds to the signal light) together with the reference light for reading. Consequently, in the reading light with which the hologram is irradiated, leak light (unnecessary component) that leaks to the region that corresponds to the signal light is significantly reduced and reproduction characteristics remarkably improve.
- the reference light with which the optical recording medium 94 has been irradiated passes through the recording layer 94 A of the optical recording medium 94 , it is diffracted by the hologram and reflected by the reflecting layer 94 B, and the transmitted diffracted light (reproduced light) is emitted toward the Fourier transform lens 92 .
- Some of the reference light is reflected by the reflecting layer 94 B of the optical recording medium 94 without being diffracted.
- the emitted reproduced light (including the reflected reference light) is inverse-Fourier-transformed by the Fourier transform lens 92 and is again converted into linearly polarized light by the 1 ⁇ 4 wavelength plate 90 .
- the reproduced light that has been converted into linearly polarized light by the 1 ⁇ 4 wavelength plate 90 is relayed and converted into parallel light by the pair of lenses 88 and 84 , is made incident on the polarizing beam splitter 24 , is reflected by the polarizing beam splitter 24 , and is made incident on the sensor array 96 .
- the sensor array 96 converts the reproduced light that has been received into electrical signals and outputs the electrical signals.
Abstract
Description
- This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-044319 filed Feb. 26, 2008.
- 1. Technical Field
- The present invention relates to an optical recording device and an optical recording and reproduction device.
- 2. Related Art
- Recently, as a holographic memory recording and reproduction method, there has been proposed the coaxial recording method (collinear method) that has advantages in that its optical system can be significantly simplified in comparison to the conventional two-beam interference method, it is resistant to external disturbance such as vibration, and the introduction of a servo mechanism is easy. In this collinear method, signal light and reference light that have been modulated and generated by a spatial light modulator are collected with the same optical axis by the same lens, and an interference fringe (diffraction grating) that is formed by the interference between the signal light and the reference light is recorded as a hologram on an optical recording medium. A signal light pattern in which digital data have been two-dimensionally encoded is displayed on the spatial light modulator, whereby the digital data are superposed on the signal light.
- The optical recording medium on which the hologram has been recorded is irradiated with the reference light as reading light, whereby the signal light is reproduced from the recorded hologram. From this reproduced signal light, the superposed digital data can be decoded. However, in the conventional collinear method, since the signal light and the reference light are on the same optical axis, when the optical recording medium is irradiated with the reference light as reading light during reproduction, light leaking to the region of the spatial light modulator that corresponds to the signal light. For example, when the precision of liquid crystal elements that have been disposed as a transmissive spatial light modulator is low, the light that has been transmitted through the OFF pixels that are in display positions of the signal light pattern becomes leak light. Consequently, when all of the pixels of the spatial light modulator are irradiated with light in this manner, this leak light is detected by a photodetector, noise with respect to the reproduced signal light is generated, and reproduction characteristics deteriorate.
- According to an aspect of the invention, an optical recording device comprises a light source, a spatial light modulator, a correcting optical system, and a light guiding section. The light source emits coherent light. The spatial light modulator is formed from a plurality of pixels that are two-dimensionally arrayed, comprises a signal light region that displays a signal light pattern when generating signal light, and a reference light region that is disposed so as to surround the signal light region and displays a reference light pattern when generating reference light that is coaxial with the signal light, and modulates and outputs incident light for each pixel in accordance with a display pattern. The correcting optical system is disposed between the light source and the spatial light modulator, and comprises a pair of axicon lenses that correct the light that has been emitted from the light source so as to flatten the light intensity distribution thereof on an irradiated surface of the spatial light modulator. The light guiding section guides the light that has been corrected by the correcting optical system to the spatial light modulator. The signal light and the reference light generated by the spatial light modulator are irradiated onto an optical recording medium simultaneously and a hologram is recorded into the optical recording medium.
- An exemplary embodiment of the present invention will be described in detail on the basis of the attached drawings, wherein:
-
FIG. 1 is a schematic diagram showing the configuration of an optical recording and reproduction device pertaining to the exemplary embodiment of the present invention; -
FIG. 2A is a diagram showing an example of a recording pattern that is displayed on a spatial light modulator during recording; -
FIG. 2B is a plane view showing an example of a display region that is set on a display screen of the spatial light modulator; -
FIG. 3A is a schematic diagram showing an example of the configuration of an axicon optical system; -
FIG. 3B andFIG. 3C are schematic diagrams showing an example of a diaphragm mechanism; -
FIG. 4A is a diagram showing beam correction operation of the axicon optical system during recording; -
FIG. 4B is a diagram showing beam correction operation of the axicon optical system during reproduction; -
FIG. 5A is a diagram for describing an optical condition where circular incident parallel light with a diameter D is emitted as circular parallel light with the diameter D; -
FIG. 5B is a diagram showing a result of simulating a light intensity distribution being flattened by a pair of axicon lenses; -
FIG. 6A toFIG. 6D are diagrams showing results when the effect of flattening the light intensity distribution by the pair of axicon lenses has been simulated; -
FIG. 7 is a diagram showing the arrangement of the respective lenses that configure the pair of axicon lenses; -
FIG. 8A is a perspective diagram showing the display surface of the spatial light modulator being irradiated with laser light during recording; -
FIG. 8B is a perspective diagram showing the display surface of the spatial light modulator being irradiated with laser light during reproduction; -
FIG. 9A andFIG. 9B are diagrams showing the effect of introducing the axicon optical system; -
FIG. 10A is a histogram of ON pixels (white pixels) of a data page shown inFIG. 9A ; -
FIG. 10B is a histogram of ON pixels of a data page shown inFIG. 9B ; and -
FIG. 11 is a schematic diagram showing the configuration of a coaxial reflective optical recording and reproduction device to which the axicon optical system has been introduced. - Below, an example of an exemplary embodiment of the present invention will be described in detail with reference to the drawings.
- General Configuration of Optical Recording and Reproduction Device
-
FIG. 1 is a schematic diagram showing the configuration of an optical recording and reproduction device pertaining to the exemplary embodiment of the present invention. This optical recording and reproduction device is a “coaxial recording method (collinear method)” optical recording and reproduction device that irradiates an optical recording medium with signal light and reference light that share an optical axis as recording light of one light beam from the same direction. In the present exemplary embodiment, a “coaxial transmissive” optical recording and reproduction device that uses a reflective spatial light modulator (SLM) and a transmissive optical recording medium will be described. - In the optical recording and reproduction device, there is disposed a
light source 10 that emits laser light that is coherent light. As thelight source 10, for example, a laser light source that emits green laser light whose emission wavelength is 532 nm is used. On the light exiting side of thelight source 10, ashutter 12 that is capable of inserting into and withdrawing from (opening and closing) the optical path, a ½wavelength plate 14 that applies a phase difference of ½ wavelength between linearly polarized components that are orthogonal, apolarizing plate 16 that allows light of a predetermined polarization direction to pass therethrough, abeam expander 18 that is an expanding/collimating optical system, an axiconoptical system 20 that is used as a correcting optical system and a reflectingmirror 22 are disposed in this order along the optical path from the side of thelight source 10. Theshutter 12 is driven to open and close by a drive device (not shown) that is connected to a control device (not shown) such as a computer. - On the light reflecting side of the reflecting
mirror 22, there is disposed apolarizing beam splitter 24 that reflects light of a predetermination polarization direction and transmits light of a polarization direction that is orthogonal thereto. When seen from the reflectingmirror 22 side, on the light reflecting side of thepolarizing beam splitter 24, there is disposed a reflective spatiallight modulator 26 that polarizes and modulates incident light per pixel. As the reflective spatiallight modulator 26, there can be used liquid crystal on silicon (LCOS) or the like. The spatiallight modulator 26 is connected to a control device (not shown) via a pattern generator (not shown). Each of the pixel portions of the spatiallight modulator 26 is driven and controlled by this control device. - The pattern generator applies two-dimensional encoding to digital data that have been supplied from the control device to generate a signal light pattern that is displayed on the spatial
light modulator 26. The signal light pattern is, for example, a digital pattern where binary digital data “0, 1” are expressed as “dark (black pixels), bright (white pixels)”. On the spatiallight modulator 26, in addition to the signal light pattern, a reference light pattern is also displayed. The reference light pattern is, for example, a random pattern. The spatiallight modulator 26 modulates incident laser light in accordance with the displayed signal light pattern and reference light pattern to generate signal light and reference light. The spatiallight modulator 26 reflects the generated signal light and reference light toward thepolarizing beam splitter 24. -
FIG. 2A is a diagram showing an example of a recording pattern that is displayed on the spatiallight modulator 26 during recording. As shown inFIG. 2A , arecording pattern 64 includes asignal light pattern 66 that generates signal light and a ring-shapedreference light pattern 68 that generates reference light. Thesignal light pattern 66 is displayed in the central portion of the spatiallight modulator 26. Thereference light pattern 68 is displayed on the peripheral portion of the spatiallight modulator 26 so as to surround thissignal light pattern 66. The region that displays the signal light pattern is a signal light region, and the region that displays the reference light pattern is a reference light region. The shape of the signal light region and the shape of the reference light region can be appropriately altered in accordance with the recording pattern. -
FIG. 2B is a plane view showing an example of a display region that is set on adisplay screen 26A of the spatiallight modulator 26. The size of the recording pattern and the pattern disposition are set beforehand in accordance with the size and the like of thedisplay screen 26A. For example, in the present exemplary embodiment, as shown inFIG. 2B , on thedisplay screen 26A of the spatiallight modulator 26, there are respectively disposed a circularsignal light region 26S and a ring-shaped referencelight region 26R that surrounds thesignal light region 26S. - The axicon
optical system 20 is a correcting optical system at least which, during recording, corrects incident parallel light from thebeam expander 18 so as to flatten its light intensity distribution on thedisplay screen 26A of the spatiallight modulator 26 and generates parallel light with which the spatiallight modulator 26 is irradiated. Further, during reproduction, the axiconoptical system 20 corrects incident parallel light from thebeam expander 18 such that thesignal light region 26S of the spatiallight modulator 26 is not irradiated with laser light and generates parallel light with which the spatiallight modulator 26 is irradiated. - In the present exemplary embodiment, during recording, the axicon
optical system 20 generates parallel light whose cross-sectional shape that is orthogonal to the optical path is circular (below, called “circular parallel light”) in order to irradiate the circular recording region that includes thesignal light region 26S and thereference light region 26R with laser light. Further, during reproduction, the axiconoptical system 20 generates parallel light whose cross-sectional shape that is orthogonal to the optical path is ring-shaped (below, called “ring-shaped parallel light”) in order to irradiate the ring-shaped referencelight region 26R with laser light. It will be noted that the axiconoptical system 20 will be described in detail later. - On the
polarizing beam splitter 24, the signal light and the reference light that have been generated by the spatiallight modulator 26 are made incident. When seen from this spatiallight modulator 26, on the light transmitting side of thepolarizing beam splitter 24, a pair oflenses Fourier transform lens 34 are disposed in this order along the optical path. TheFourier transform lens 34 Fourier-transforms recording light and irradiates anoptical recording medium 36 with the Fourier-transformed recording light. The focal position of theFourier transform lens 34 becomes a light collecting position where the recording light is collected. Between thelens 28 and thelens 32, in the vicinity of the beam waist, there is disposed alight blocking plate 30 that includes an opening portion (an aperture) 30A. It will be noted that thelight blocking plate 30 is not essential and can be appropriately omitted. - On the light exiting side of the
Fourier transform lens 34, there is disposed a holding stage (not shown) that holds theoptical recording medium 36. The holding stage is driven by a drive device (not shown) that is connected to a control device (not shown) and moves in the optical axis direction or a plane direction that is perpendicular to the optical axis. The holding stage, for example, holds theoptical recording medium 36 in a standard position where the film-thickness direction center position of theoptical recording medium 36 becomes the focal position of theFourier transform lens 34. - The
optical recording medium 36 is an optical recording medium on which a hologram is capable of being recorded by a change in the index of refraction resulting from being irradiated with light. Examples of theoptical recording medium 36 may include an optical recording medium that uses a recording material such as a photopolymer material, a photorefractive material or a silver halide photosensitive material. - On the light transmitting side of the
optical recording medium 36, there are disposed aFourier transform lens 38, a pair oflenses sensor array 46. Thesensor array 46 is configured by imaging elements such as CCDs or a CMOS array, converts reproduced light (diffracted light) that has been received into electrical signals and outputs the electrical signals. Between thelens 38 and thelens 42, there is disposed alight blocking plate 40 that includes an opening portion (an aperture) 40A with a large opening diameter. It will be noted that thelight blocking plate 40 is not essential and can be appropriately omitted. Further, thesensor array 46 is connected to a control device (not shown). During reproduction, thesensor array 46 reads data that have been superposed on signal light that has been reproduced and outputs the data to a control device (not shown). - Configuration of Axicon Optical System
- Next, the configuration of the axicon
optical system 20 will be described.FIG. 3A is a schematic diagram showing an example of the configuration of the axiconoptical system 20. The axiconoptical system 20 is configured to include amask 50 that comprises a light transmitting plate and includes a blockingportion 50A that blocks light in the vicinity of an optical axis L, a diaphragm mechanism (an iris) 56 that includes anopening portion 56A whose diameter changes to adjust the beam diameter of the light that passes therethrough, and a pair ofaxicon lenses 58. Laser light is made incident on the axiconoptical system 20 from thebeam expander 18. Themask 50, thediaphragm mechanism 56 and the pair ofaxicon lenses 58 are disposed in this order along the optical path from the laser light incident side. - The
diaphragm mechanism 56 can, as shown inFIG. 3B andFIG. 3C , for example, be configured by superimposing plural plates (aperture blades) 56B. Each of theplural aperture blades 56B changes the diameter of theopening portion 56A by fixing and simultaneously rotating one end. As shown inFIG. 3B , during recording, in a state where the diameter of theopening portion 56A has expanded, light with a large opening diameter is allowed to pass therethrough. On the other hand, as shown inFIG. 3C , during reproduction, the diameter of theopening portion 56A contracts toward the optical axis from the outside along the direction of arrow A such that the beam diameter of the light that passes through theopening portion 56A becomes smaller. It will be noted that, when an automatic diaphragm mechanism or the like that uses an ultrasonic motor is used as thediaphragm mechanism 56, high-speed driving becomes possible. -
FIG. 7 is a diagram showing the arrangement of the respective lenses that configure the pair ofaxicon lenses 58. The pair ofaxicon lenses 58 are configured by two lenses that have the same shape: anaxicon lens 58A and anaxicon lens 58B. An axicon is an optical device that is disposed with an axisymmetric surface that converts light from a point light source into a linear image on an optical axis. There are many instances where an axicon is used in the configuration of a conical lens that is disposed with a conical surface, and a truncated cone-shaped lens where one surface is a flat surface and the other surface is a conical surface is usually called an axicon lens. In the present exemplary embodiment also, the truncated cone-shaped lenses shown inFIG. 7 are called axicon lenses. Theaxicon lens 58A and theaxicon lens 58B are disposed apart from each other by a predetermined distance such that their conical axes coincide with the optical axis L (i.e., such that each of the apexes of the cones is positioned on the same optical axis) and such that their conical surfaces face each other. The distance by which theaxicon lens 58A and theaxicon lens 58B are disposed apart from each other will be described in detail later. - Beam Correction Operation of Axicon Optical System Next, operation of the axicon
optical system 20 shown inFIG. 3A will be described.FIG. 4A is a diagram showing beam correction operation of the axiconoptical system 20 during recording, andFIG. 4B is a diagram showing beam correction operation of the axiconoptical system 20 during reproduction. - First, beam correction operation during recording will be described. As shown in
FIG. 4A , on the axiconoptical system 20, circular parallel light with a diameter D is made incident from thebeam expander 18. Themask 50 that includes the blockingportion 50A is irradiated with this circular parallel light with the diameter D. As for the parallel light with which themask 50 has been irradiated, the light intensity component that would be transmitted through the apexes of theaxicon lenses portion 50A, and the remaining portion passes through themask 50. Thediaphragm mechanism 56 is irradiated with the parallel light that has passed through themask 50. During recording, the diameter of theopening portion 56A in thediaphragm mechanism 56 is in an expanded state. The circular parallel light with the diameter D passes as is through theopening portion 56A. - The light intensity distribution of the circular parallel light with the diameter D that has passed through the
opening portion 56A in thediaphragm mechanism 56 is flattened by theaxicon lenses 58, and the circular parallel light with the diameter D is emitted as circular parallel light 100 with the same diameter D. That is, when the circular parallel light with the diameter D is made incident from the flat surface side of theaxicon lens 58A, when the incident light is emitted from the slanted surface and the apex of the conical surface, it is refracted as inFIG. 4A and a circular beam is formed. The circular beam is made incident from the conical surface of theaxicon lens 58B, is formed into the circular parallel light 100 with the diameter D, and is emitted from the flat surface side of theaxicon lens 58B. That is, it fulfills the role of folding back the light in the vicinity of the optical axis of the incident light to the outside and of folding back the light in the vicinity of the circumference to the vicinity of the optical axis. Laser light has a Gaussian intensity distribution that has a peak on its optical axis, so by spreading the light in the vicinity of the optical axis where its intensity is high to the outside and causing the light in the vicinity of the circumference where its intensity is low to be concentrated in the vicinity of the optical axis, it becomes possible to make the light intensity distribution uniform. - In the present exemplary embodiment, the diameter D of the circular
parallel light 100 is designed so as to become equal to the outer diameter of thereference light region 26R. Thus, thesignal light region 26S and thereference light region 26R shown inFIG. 2B are exactly irradiated with the circularparallel light 100. - Next, beam correction operation during reproduction will be described. As shown in
FIG. 4B , themask 50 that includes the blockingportion 50A is irradiated with the circular parallel light with the diameter D that has been made incident on the axiconoptical system 20. As for the parallel light with which themask 50 has been irradiated, the light intensity component that would be transmitted through the apexes of theaxicon lenses portion 50A, and the remaining portion passes through themask 50. Thediaphragm mechanism 56 is irradiated with the parallel light that has passed through themask 50. During reproduction, the diameter of theopening portion 56A in thediaphragm mechanism 56 contracts. The diameter of theopening portion 56A at this time is d (<D). The outside of the circular parallel light with the diameter D with which thediaphragm mechanism 56 has been irradiated is cut by thediaphragm mechanism 56 such that just circular parallel light with the diameter d passes through theopening portion 56A. - The circular parallel light with the diameter d that has passed through the
opening portion 56A in thediaphragm mechanism 56 is converted from a circular beam into a ring-shaped beam by the pair ofaxicon lenses 58 such that ring-shapedparallel light 102 is emitted. As shown also inFIG. 7 , when the circular parallel light with the diameter d is made incident from the flat surface side of theaxicon lens 58A, when the incident light is emitted from the slanted surface and the apex of the conical surface, it is refracted as inFIG. 4B and a ring-shaped beam is formed. The ring-shaped beam is made incident from the conical surface of theaxicon lens 58B, is formed into the ring-shapedparallel light 102, and is emitted from the flat surface side of theaxicon lens 58B. - The outer diameter (diameter of the outer circumference) of the ring-shaped
parallel light 102 that is emitted is D, and the inner diameter (diameter of the inner circumference) of the ring-shapedparallel light 102 that is emitted is Din (seeFIG. 7 ). The outer diameter D and the inner diameter Din are determined in accordance with the diameter d of the incident parallel light, a diameter A of the axicon lenses, an angle φ of the apex angle of the conical portions and a distance L by which the pair of axicon lenses are disposed apart from each other. The distance L by which the pair of axicon lenses are disposed apart from each other will be described later. - In the present exemplary embodiment, the outer diameter D and the inner diameter Din of the ring-shaped
parallel light 102 are designed such that the outer diameter D becomes equal to the outer diameter of thereference light region 26R and such that the inner diameter Din becomes equal to the inner diameter of thereference light region 26R. Thus, thereference light region 26R shown inFIG. 2B is exactly irradiated with the ring-shapedparallel light 102. - It will be noted that, in the present exemplary embodiment, the inner diameter of the
reference light region 26R and the diameter of thesignal light region 26S are described as being equal, but from the standpoint of reducing leak light that leaks to the region that corresponds to the signal light during reproduction, it is preferable to make the inner diameter of thereference light region 26R larger than the diameter of thesignal light region 26S. That is, it is preferable to dispose a gap between thereference light region 26R and thesignal light region 26S. Further, from the same standpoint, it is more preferable to make the inner diameter Din of the ring-shapedparallel light 102 slightly larger than the inner diameter of thereference light region 26R. -
FIG. 5A is a diagram for describing an optical condition for the circular incident parallel light with the diameter D to be emitted as circular parallel light with the same diameter D as shown inFIG. 4A . As shown inFIG. 5A , the distance L by which theaxicon lens 58A and theaxicon lens 58B are disposed apart from each other is defined as an optical path component distance of the optical path length between theaxicon lens 58A and theaxicon lens 58B. - Assuming that φ represents the angle of the apex angle of the axicon lenses, D represents the beam diameter of the incident light, D represents the beam diameter of the light that exits, and θ represents the angle of refraction resulting from the axicon lenses, then the distance L by which the axicon lenses are disposed apart from each other is expressed by the following expression.
-
- Assuming that n represents the index of refraction of the axicon lenses and that 1 represents the index of refraction in air, then the relationship of the following expression is established from Snell's law.
-
- From the above expressions, in order for the circular parallel light with the diameter D that has been made incident on the
axicon lens 56A to be emitted as circular parallel light with the same diameter D from theaxicon lens 58B, it is necessary for theaxicon lens 58A and theaxicon lens 58B to be disposed apart from each other by the distance L that is expressed by the following expression (1). -
- Further,
FIG. 5B is a diagram showing a result of simulating incident light being refracted by the pair ofaxicon lenses 58 with a wave-optics technique. Arrow B represents the waveguide direction of the light. As shown inFIG. 5B , it will be understood that, in incident parallel light having a Gaussian intensity distribution, the light in the vicinity of the optical axis is folded back in the outside circumference direction and the light in the vicinity of the circumference is folded back in the vicinity of the optical axis. Thus, the light intensity distribution is converted while the parallel light passes through the pair ofaxicon lenses 58, and parallel light is generated whose light intensity distribution in its cross section that is orthogonal to the optical axis is uniform. -
FIG. 6A toFIG. 6D are diagrams showing results when the effect of flattening the light intensity distribution by the pair of axicon lenses has been simulated with a wave-optics technique. Here, theaxicon lens 58A and theaxicon lens 58B are disposed apart from each other by the distance L so as to satisfy the above expression. Further, the above-describedmask 50 is not used. -
FIG. 6A andFIG. 6C are diagrams showing the beam profile of the parallel light before being made incident on the pair ofaxicon lenses 58.FIG. 6B andFIG. 6D are diagrams showing the beam profile of the parallel light that has been emitted from the pair ofaxicon lenses 58. InFIG. 6C andFIG. 6D , the horizontal axis represents distance (unit: mm) from a standard position, and the optical axis is in a position of 7 mm from the standard position. The vertical axis represents a relative light intensity A when a maximum light intensity is 1. - As shown in
FIG. 6A andFIG. 6C , in the beam that has a Gaussian intensity distribution, the light intensity in the central portion (in the vicinity of the optical axis) is large, and the light intensity decreases toward the periphery. In contrast, in the beam that has passed through the pair ofaxicon lenses 58, as shown inFIG. 6B andFIG. 6D , the light intensity in the central portion is distributed to the peripheral portion, the light intensity at the peripheral portion increases, and the light intensities are substantially the same between the central portion and the peripheral portion. The beam profile shown inFIG. 6D has a substantially trapezoidal shape. That is, as a result of passing through the pair ofaxicon lenses 58, the beam that has a Gaussian intensity distribution is converted into a beam whose light intensity is flattened and whose light intensity distribution in its cross section that is orthogonal to the optical axis is uniform. - However, as will be understood from
FIG. 6B andFIG. 6D , when the light intensity distribution is simply flattened using the pair ofaxicon lenses 58, turbulence in the light intensity distribution arises in the central portion even after the light intensity distribution has been flattened. The cause of this is diffraction at the apexes of the axicon lenses. Consequently, as mentioned above, themask 50 that includes the blockingportion 50A is disposed on the light incident side of the pair ofaxicon lenses 58 to thereby block the light intensity component that would be transmitted through the apexes of theaxicon lenses 58 and travel straightly. Thus, the turbulence in the light intensity distribution is removed at the central portion, and the light intensity distribution is made even more uniform. However, themask 50 can also be appropriately omitted by a contrivance such as not using the signal light region in the vicinity of the center where the light intensity distribution becomes turbulent. It will be noted that the conditional expression in the present invention has been described by a case where themask 50 is omitted. When themask 50 is introduced, the conditional expression may be appropriately corrected in accordance with the size of the blockingportion 50A. - As described above, during recording, the axicon
optical system 20 generates the circular parallel light 100 with the diameter D whose light intensity distribution has been flattened in order to irradiate, with laser light, the circular recording region that includes thesignal light region 26S and thereference light region 26R shown inFIG. 2B . Further, during reproduction, the axiconoptical system 20 generates ring-shaped parallel light in order to irradiate, with laser light, just the ring-shaped referencelight region 26R of the spatiallight modulator 26. That is, during reproduction, the axiconoptical system 20 generates ring-shaped parallel light such that thesignal light region 26S of the spatiallight modulator 26 is not irradiated with laser light. - It will be noted that the parallel light generated by the axicon
optical system 20 is, as mentioned later, reflected by the reflectingmirror 22 and reflected by thepolarizing beam splitter 24 such that the spatiallight modulator 26 is irradiated therewith. At this time, as mentioned above, during recording, thesignal light region 26S and thereference light region 26R are irradiated with the circularparallel light 100 and, during reproduction, thereference light region 26R is irradiated with the ring-shapedparallel light 102. - Recording/Reproduction Operation of Optical Recording and Reproduction Device
- Next, operation of recording/reproduction of the optical recording and reproduction device shown in
FIG. 1 will be described. - When a hologram is to be recorded, the
shutter 12 is opened and laser light is emitted from thelight source 10. At the same time, a recording pattern is displayed on the spatiallight modulator 26. The laser light that has been emitted from thelight source 10 passes through theshutter 12, and its light intensity and polarization direction are adjusted by the ½wavelength plate 14 and thepolarizing plate 16. For example, thepolarizing plate 16 has an arrangement that transmits only S-polarized light, and the polarization direction of the laser light is controlled by the ½wavelength plate 14, whereby the light intensity of the S-polarized light is adjusted. The light that has passed through thepolarizing plate 16 is converted into parallel light with a large diameter by thebeam expander 18, and circular parallel light with the diameter D is made incident on the axiconoptical system 20. - In the axicon
optical system 20, the circular incident parallel light with the diameter D is corrected, and the circular parallel light 100 with the diameter D whose light intensity distribution has been flattened is generated. The reflectingmirror 22 is irradiated with the parallel light that has been emitted from the axiconoptical system 20. The parallel light that has been reflected by the reflectingmirror 22 is made incident on thepolarizing beam splitter 24. Here, thepolarizing beam splitter 24 reflects S-polarized light and transmits P-polarized light. The circular parallel light 100 with the diameter D (S-polarized light) is reflected by thepolarizing beam splitter 24 in the direction of the spatiallight modulator 26. On the spatiallight modulator 26, the recording pattern is displayed. In the spatiallight modulator 26, the laser light is polarized and modulated (from S-polarized light to P-polarized light) in accordance with the displayed pattern, and signal light and reference light are generated. - In the present exemplary embodiment, as shown in
FIG. 8A , thesignal light region 26S and thereference light region 26R of the spatiallight modulator 26 are irradiated with the circularparallel light 100 generated by the axiconoptical system 20. The laser light that has been made incident on thesignal light region 26S is polarized and modulated in accordance with the displayed signal light pattern, and signal light is generated. Further, the laser light that has been made incident on thereference light region 26R is polarized and modulated in accordance with the displayed reference light pattern, and reference light is generated. Because the light intensity distribution of the circularparallel light 100 is flattened, the recording pattern that is displayed on the spatiallight modulator 26 can be irradiated uniformly. Thus, the SNR of the signal light that is generated improves, and a hologram is recorded in a high SNR. - The recording light that has been polarized and modulated by the spatial
light modulator 26 is emitted to thepolarizing beam splitter 24, passes through thepolarizing beam splitter 24, and is converted into an amplitude distribution of linearly polarized light (P-polarized light). Thereafter, the recording light is collected by thelens 28, and thelight blocking plate 30 that includes theaperture 30A is irradiated therewith. The unnecessary frequency component of the recording light that has been collected by thelens 28 is cut by thelight blocking plate 30, and the remaining portion passes through theaperture 30A. The recording light that has passed through theaperture 30A is converted into parallel light by thelens 32. - The recording light that has been converted into parallel light by the
lens 32, that is, the signal light and the reference light are Fourier-transformed and collected by theFourier transform lens 34, and theoptical recording medium 36 is simultaneously and coaxially irradiated therewith. In the position where the signal light and the reference light are collected, an interference fringe that is formed as a result of the signal light and the reference light interfering with each other is recorded as a hologram on theoptical recording medium 36. - When data that have been recorded on the
optical recording medium 36 are to be read (during reproduction), theshutter 12 is opened and laser light is emitted from the light source. At the same time, a reproduction pattern is displayed on the spatiallight modulator 26. The laser light that has been emitted from thelight source 10, in the same manner as in the case of recording, passes through theshutter 12, its light intensity and polarization direction are adjusted by the ½wavelength plate 14 and thepolarizing plate 16, it is converted into parallel light with a large diameter by thebeam expander 18, and it is made incident on the axiconoptical system 20. - In the axicon
optical system 20, the circular incident parallel light with the diameter D is corrected, and the ring-shaped parallel light 102 with the outer diameter D and the inner diameter d is generated. The light intensity distribution of the ring-shapedparallel light 102 is also flattened. The reflectingmirror 22 is irradiated with the ring-shapedparallel light 102 that has been emitted from the axiconoptical system 20. The ring-shapedparallel light 102 that has been reflected by the reflectingmirror 22 is made incident on thepolarizing beam splitter 24. The ring-shaped incident parallel light 102 (S-polarized light) is reflected by thepolarizing beam splitter 24 in the direction of the spatiallight modulator 26. On the spatiallight modulator 26, the reproduction pattern is displayed. In the spatiallight modulator 26, the laser light is polarized and modulated (from S-polarized light to P-polarized light) in accordance with the displayed pattern, and reference light is generated. - In the present exemplary embodiment, as shown in
FIG. 8B , thereference light region 26R of the spatiallight modulator 26 is irradiated with the ring-shapedparallel light 102 that has been generated by the axiconoptical system 20. The laser light that has been made incident on thereference light region 26R is polarized and modulated in accordance with the displayed reference light pattern, and reference light is generated. The region of theoptical recording medium 36 on which the hologram has been recorded is, in the same manner as in the case of recording, irradiated with the generated reference light that. That is, theoptical recording medium 36 is irradiated with just the reference light as reading light. - The ring-shaped
parallel light 102 is generated in accordance with the shape of thereference light region 26R, just thereference light region 26R is irradiated with the laser light, and thesignal light region 26S is not irradiated with the laser light. Consequently, theoptical recording medium 36 is not irradiated with the light that has been reflected by thesignal light region 26S (leak light that is present in the region that corresponds to the signal light) together with the reference light for reading. Consequently, in the reading light with which the hologram is irradiated, leak light (unnecessary component) that leaks to the region that corresponds to the signal light is significantly reduced and reproduction characteristics remarkably improve. - When the reference light with which the
optical recording medium 36 has been irradiated passes through theoptical recording medium 36, it is diffracted by the hologram, and the transmitted diffracted light (reproduced light) is emitted toward theFourier transform lens 38. Some of the reference light is transmitted through theoptical recording medium 36 without being diffracted. The emitted reproduced light (including the transmitted diffracted light) is inverse-Fourier-transformed by theFourier transform lens 38, and thelight blocking plate 40 that includes theaperture 40A is irradiated therewith. As for the reproduced light that has been inverse-Fourier-transformed by thelens 38, the transmitted reference light is cut by thelight blocking plate 40 and the remaining portion passes through theaperture 40A. The light that has passed through theaperture 40A is relayed by the pair oflenses sensor array 46. - The
sensor array 46 converts the reproduced light that has been received into electrical signals and outputs the electrical signals. That is, thesensor array 46 reads the data that have been superposed on the reproduced signal light and outputs the data to a control device (not shown). It will be noted that, in thesensor array 46, it is preferable to implement oversampling where one pixel of the signal light data is received by plural light receiving elements. For example, data of 1 bit are received by four (2×2) light receiving elements. - Experimental Results
-
FIG. 9A andFIG. 9B are diagrams showing the effect of introducing the axicon optical system.FIG. 9A is a diagram showing a result when the light intensity distribution of signal light (a data page) observed on thedisplay surface 26A of the spatiallight modulator 26 has been simulated in a case where there is used an optical recording and reproduction device with the same configuration as has conventionally been the case excluding the axicon optical system from the configuration shown inFIG. 1 (below, called “the conventional device”).FIG. 9B is a diagram showing a result when the light intensity distribution of a data page observed on thedisplay surface 26A of the spatiallight modulator 26 has been simulated in a case where there is used an optical recording and reproduction device with the same configuration as the configuration shown inFIG. 1 (below, called “the device to which the axicon optical system has been introduced”). That is, the light intensity distributions of the data pages shown inFIG. 9A andFIG. 9B were obtained by multiplying the light intensity distributions shown inFIG. 6A andFIG. 6B that have been obtained by simulation and the intensity distribution of an ideal data pattern. - As will be understood from
FIG. 9A , in the conventional device to which the axicon optical system was not introduced, the spatial light modulator is irradiated with the laser light with the Gaussian intensity distribution shown inFIG. 6A , so the central portion of the data page observed is bright but the peripheral portion is dark. In contrast, as will be understood fromFIG. 9B , in the device to which the axicon optical system has been introduced, the spatial light modulator is irradiated with the laser light with the flattened light intensity distribution shown inFIG. 6B , so a data page with uniform brightness is observed. Thus, in the device to which the axicon optical system has been introduced, the spatial light modulator is uniformly irradiated with the signal light pattern that is displayed on the spatiallight modulator 26, signal light (a data page) with a high SNR is generated. -
FIG. 10A is a histogram of ON pixels (white pixels) of the data page shown inFIG. 9A , andFIG. 10B is a histogram of ON pixels of the data page shown inFIG. 9B . The horizontal axes represent brightness (in arbitrary units), and the vertical axes represent the number of occurrences (units: number of times) of ON pixels within the data pages. These histograms were obtained from the above-described simulation results. - As shown in
FIG. 10A , in the data page that was obtained in the conventional device, the brightness of the ON pixels distributes in the range of 8 to 40, and the SNR of the data page is 4.75. In contrast, as shown inFIG. 10B , in the data page that was obtained in the device to which the axicon optical system has been introduced, the SNR of the data page improves to 15.73, or greater than 3 times the SNR of the data page that was obtained in the conventional device. The fact that the range of the brightness distribution of the ON pixels is narrow means that the brightness of the ON pixels is substantially constant within the data page and that the light intensity distribution is flattened. Thus, it is apparent that, in the device to which the axicon optical system has been introduced, signal light (a data page) with a high SNR is generated. It will be noted that, here, SNR is defined as being equal to: (average value of brightness)/(standard deviation of brightness). - Further, as mentioned above, in the conventional device, during reproduction also, the
signal light region 26S and thereference light region 26R of the spatiallight modulator 26 are irradiated with laser light. In contrast, in the device to which the axicon optical system has been introduced, just thereference light region 26R of the spatiallight modulator 26 is irradiated with laser light. - Further, the minimum diffraction efficiency was evaluated using the diffraction efficiency when the bit error rate (BER) is equal to 5×10−3 as a minimum diffraction efficiency that is capable of being reproduced. The smaller the value that the minimum diffraction efficiency is, a hologram with a low diffraction efficiency can be reproduced in an excellent SNR, and reproduction characteristics improve. That is, because it is possible to reduce the recording energy of the hologram that is to be recorded, it becomes possible to multiply record more holograms, which contributes to an improvement in recording density.
- The minimum diffraction efficiency when reproduced by the device to which the axicon optical system has been introduced is 0.1%, and the minimum diffraction efficiency when reproduced by the conventional device is 1.0%. It will be understood that the minimum diffraction efficiency drops from 1.0% to 0.1% and that reproduction characteristics remarkably improve. The evaluations here were performed by simulation images that were obtained by imaging leak light of the signal light region when irradiated with the signal light and computationally adding the obtained images and the images that were obtained by simulation (
FIG. 9A andFIG. 9B ). - Other Modifications
- It will be noted that, in the preceding exemplary embodiment, an example of a “coaxial transmissive” optical recording and reproduction device that uses a reflective spatial light modulator and a transmissive optical recording medium has been described, but the same effects as those of the preceding exemplary embodiment can also be obtained even when the axicon optical system is introduced to a “coaxial reflective” optical recording and reproduction device that uses a reflective spatial light modulator and a reflective optical recording medium.
FIG. 11 is a schematic diagram showing the configuration of a coaxial reflective optical recording and reproduction device to which the axicon optical system has been introduced. The same reference numerals will be given to configural portions that are the same as those of the optical recording and reproduction device pertaining to the preceding exemplary embodiment shown inFIG. 1 . - In this optical recording and reproduction device, there is disposed the
light source 10 that emits laser light. On the light exiting side of thelight source 10, theshutter 12, the ½wavelength plate 14, thepolarizing plate 16, thebeam expander 18, the axiconoptical system 20 and the reflectingmirror 22 are disposed in this order along the optical path from the side of thelight source 10. On the light reflecting side of the reflectingmirror 22, there is disposed thepolarizing beam splitter 24. When seen from the reflectingmirror 22 side, on the light reflecting side of thepolarizing beam splitter 24, there is disposed the reflective spatiallight modulator 26. The spatiallight modulator 26 modulates incident laser light in accordance with the displayed signal light pattern and reference light pattern to generate signal light and reference light. The spatiallight modulator 26 reflects the generated signal light and reference light toward thepolarizing beam splitter 24. - The axicon
optical system 20 has the same configuration as that of the preceding exemplary embodiment. The axiconoptical system 20 is, as shown inFIG. 3A , configured to include themask 50 that includes the blockingportion 50A, thediaphragm mechanism 56 and the pair ofaxicon lenses 58. Laser light is made incident on the axiconoptical system 20 from thebeam expander 18. Themask 50, thediaphragm mechanism 56 and the pair ofaxicon lenses 58 are disposed in this order along the optical path from the laser light incident side. - On the
polarizing beam splitter 24, the signal light and the reference light that have been generated by the spatiallight modulator 26 are made incident. When seen from this spatiallight modulator 26, on the light transmitting side of thepolarizing beam splitter 24, a pair oflenses wavelength plate 90 that converts linearly polarized light into circularly polarized light and which converts circularly polarized light into linearly polarized light and aFourier transform lens 92 are disposed in this order along the optical path. TheFourier transform lens 92 irradiates a reflectiveoptical recording medium 94 with recording light. The focal position of theFourier transform lens 92 becomes a light collecting position where the recording light is collected. Between thelens 84 and thelens 88, in the vicinity of the beam waist, there is disposed alight blocking plate 86 that includes an opening portion (an aperture) 86A. It will be noted that thelight blocking plate 86 is not essential and can be appropriately omitted. - On the light exiting side of the
Fourier transform lens 92, there is disposed a holding stage (not shown) that holds theoptical recording medium 94. Theoptical recording medium 94 is an optical recording medium on which a hologram is capable of being recorded by a change in the index of refraction resulting from being irradiated with light, and theoptical recording medium 94 is disposed with arecording layer 94A that is configured by a recording material such as a photopolymer material on which a hologram is capable of being recorded and areflective layer 94B that is configured by a metal film or the like that reflects light that is transmitted through therecording layer 94A. - When seen from the
lens 84, on the light reflecting side of thepolarizing beam splitter 24, there is disposed asensor array 96. Thesensor array 96 is configured by imaging elements such as CCDs or a CMOS array, converts reproduced light (diffracted light) that has been received into electrical signals and outputs the electrical signals. - When a hologram is to be recorded, the
shutter 12 is opened and laser light is emitted from thelight source 10. At the same time, a recording pattern is displayed on the spatiallight modulator 26. The laser light that has been emitted from thelight source 10 passes through theshutter 12, and its light intensity and polarization direction are adjusted by the ½wavelength plate 14 and thepolarizing plate 16. The light that has passed through thepolarizing plate 16 is converted into parallel light with a large diameter by thebeam expander 18 and is made incident on the axiconoptical system 20. - In the axicon
optical system 20, the circular incident parallel light with the diameter D is corrected, and the circular parallel light 100 with the diameter D whose light intensity distribution has been flattened is generated. The reflectingmirror 22 is irradiated with the parallel light that has been emitted from the axiconoptical system 20. The parallel light that has been reflected by the reflectingmirror 22 is made incident on thepolarizing beam splitter 24. The circular parallel light 100 with the diameter D is reflected by thepolarizing beam splitter 24 in the direction of the spatiallight modulator 26. In the spatiallight modulator 26, the laser light is polarized and modulated in accordance with the displayed pattern, and signal light and reference light are generated. - The recording light that has been polarized and modulated by the spatial
light modulator 26 is emitted to thepolarizing beam splitter 24, passes through thepolarizing beam splitter 24, and is converted into an amplitude distribution of linearly polarized light. Thereafter, the recording light is collected by thelens 84, and thelight blocking plate 86 that includes theaperture 86A is irradiated therewith. The unnecessary frequency component of the recording light that has been collected by thelens 84 is cut by thelight blocking plate 86, and the remaining portion passes through theaperture 86A. The recording light that has passed through theaperture 86A is converted into parallel light by thelens 88. - The recording light (the signal light and the reference light) that has been converted into parallel light by the
lens 88 is converted into circularly polarized light by the ¼wavelength plate 90 and is Fourier-transformed and collected by theFourier transform lens 92, and theoptical recording medium 94 is simultaneously and coaxially irradiated therewith. In the position where the signal light and the reference light are collected, an interference fringe that is formed as a result of the signal light and the reference light interfering with each other is recorded as a hologram on theoptical recording medium 94. - When data that have been recorded on the
optical recording medium 94 are to be read, theshutter 12 is opened and laser light is emitted from the light source. At the same time, a reproduction pattern is displayed on the spatiallight modulator 26. The laser light that has been emitted from thelight source 10, in the same manner as in the case of recording, passes through theshutter 12, its light intensity and polarization direction are adjusted by the ½wavelength plate 14 and thepolarizing plate 16, it is converted into parallel light with a large diameter by thebeam expander 18, and it is made incident on the axiconoptical system 20. - In the axicon
optical system 20, the circular incident parallel light with the diameter D is corrected, and the ring-shaped parallel light 102 with the outer diameter D and the inner diameter d is generated. The light intensity distribution of the ring-shapedparallel light 102 is also flattened. The reflectingmirror 22 is irradiated with the ring-shapedparallel light 102 that has been emitted from the axiconoptical system 20. The ring-shapedparallel light 102 that has been reflected by the reflectingmirror 22 is made incident on thepolarizing beam splitter 24. The ring-shaped incidentparallel light 102 is reflected by thepolarizing beam splitter 24 in the direction of the spatiallight modulator 26. In the spatiallight modulator 26, the laser light is polarized and modulated in accordance with the displayed reproduction pattern, and reference light is generated. The region of theoptical recording medium 94 on which the hologram has been recorded is, in the same manner as in the case of recording, irradiated with the generated reference light. That is, theoptical recording medium 94 is irradiated with just the reference light as reading light. - The ring-shaped
parallel light 102 is generated in accordance with the shape of thereference light region 26R, just thereference light region 26R is irradiated with the laser light, and thesignal light region 26S is not irradiated with the laser light. Consequently, theoptical recording medium 94 is not irradiated with the light that has been reflected by thesignal light region 26S (leak light that is present in the region that corresponds to the signal light) together with the reference light for reading. Consequently, in the reading light with which the hologram is irradiated, leak light (unnecessary component) that leaks to the region that corresponds to the signal light is significantly reduced and reproduction characteristics remarkably improve. - When the reference light with which the
optical recording medium 94 has been irradiated passes through therecording layer 94A of theoptical recording medium 94, it is diffracted by the hologram and reflected by the reflectinglayer 94B, and the transmitted diffracted light (reproduced light) is emitted toward theFourier transform lens 92. Some of the reference light is reflected by the reflectinglayer 94B of theoptical recording medium 94 without being diffracted. The emitted reproduced light (including the reflected reference light) is inverse-Fourier-transformed by theFourier transform lens 92 and is again converted into linearly polarized light by the ¼wavelength plate 90. The reproduced light that has been converted into linearly polarized light by the ¼wavelength plate 90 is relayed and converted into parallel light by the pair oflenses polarizing beam splitter 24, is reflected by thepolarizing beam splitter 24, and is made incident on thesensor array 96. Thesensor array 96 converts the reproduced light that has been received into electrical signals and outputs the electrical signals. - The foregoing description of the exemplary embodiments of the present invention has been provided for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
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CN (1) | CN101520636B (en) |
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US20100220570A1 (en) * | 2009-02-27 | 2010-09-02 | Toshiki Ishii | Signal quality evaluating apparatus and method, and information recording medium |
US10642172B2 (en) | 2018-05-14 | 2020-05-05 | Asml Netherlands B.V. | Illumination source for an inspection apparatus, inspection apparatus and inspection method |
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- 2008-02-26 JP JP2008044319A patent/JP2009205711A/en not_active Withdrawn
- 2008-11-06 US US12/266,285 patent/US8050164B2/en not_active Expired - Fee Related
- 2008-12-09 CN CN2008101838222A patent/CN101520636B/en not_active Expired - Fee Related
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US7639587B2 (en) * | 2005-11-29 | 2009-12-29 | Canon Kabushiki Kaisha | Optical information recording and reproducing apparatus and optical information recording apparatus |
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US20100220570A1 (en) * | 2009-02-27 | 2010-09-02 | Toshiki Ishii | Signal quality evaluating apparatus and method, and information recording medium |
US8184514B2 (en) * | 2009-02-27 | 2012-05-22 | Hitachi Consumer Electronics Co., Ltd. | Signal quality evaluating apparatus and method, and information recording medium |
US10642172B2 (en) | 2018-05-14 | 2020-05-05 | Asml Netherlands B.V. | Illumination source for an inspection apparatus, inspection apparatus and inspection method |
US11347155B2 (en) | 2018-05-14 | 2022-05-31 | Asml Netherlands B.V. | Illumination source for an inspection apparatus, inspection apparatus and inspection method |
Also Published As
Publication number | Publication date |
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CN101520636B (en) | 2012-05-30 |
CN101520636A (en) | 2009-09-02 |
JP2009205711A (en) | 2009-09-10 |
US8050164B2 (en) | 2011-11-01 |
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